The development of drug resistance to antineoplastic drugs is a tremendous obstacle to effective treatment of human malignancies. The problem of drug re-sistance is compounded by the emergence of tumors cross-resistant to multiple chemotherapeutic agents related in structure and function to the original selective agent. This phenomenon, termed multidrug resistance, may be caused by a protein, P-glycoprotein, which acts as an ATP-dependent efflux pump reducing the intracellular accumulation of certain antineoplastic agents.